This invention relates to a thick film resistor of the type that is formed by applying a powder paste and sintering. More particularly, this invention relates to such resistor formed of a predominantly palladium film containing boron nitride to increase electrical resistance.
In the manufacture of electronic components, thick film resistors are formed by applying a powder paste to a substrate and heating to sinter the powder to form a film. In a typical example, the paste comprises a mixture of metallic and nonmetallic powders dispersed in a liquid vehicle that permits the paste to be conveniently applied, for example, by spraying or screen printing. The vehicle contains an expendable binder that holds the particles into a layer after the paste is applied and dried. Upon heating, the binder decomposes, and the powders are sintered to produce an integral film and to bond the film to the substrate, thereby forming the resistor.
It is also advantageous to concurrently form a thick film element and a substrate by a co-firing process. For this purpose, the paste is applied to a substrate that is a compact of ceramic powder typically bonded by an expendable organic binder. Thereafter, during heating, the ceramic is sintered into an integral substrate, while the particulate layer is concurrently sintered to form the element.
Common commercial paste for forming thick film resistors include a mixture of silver powder and one or more powders, of a glass or ceramic composition. Such silver powder pastes are limited to sintering temperatures less than about 1,000.degree. C. It is desired to utilize substrates formed of alkaline earth metal titanate compounds, such as strontium calcium titanate (SCT), that require relatively high sintering temperatures greater than about 1,250.degree. C. Thus, silver powder pastes are not suitable for co-sintering with metal titanates at the higher temperatures. Moreover, additives formed of glass or other ceramics that are suitable at silver sintering temperatures may not be useful for sintering at the higher temperatures, particularly if added in relatively high concentrations to modify film resistance. Therefore, there is a need for an electrically resistive film for forming thick film resistors and the like, which film is derived from a paste and is sintered at a high temperature such as is encountered in a co-firing process that forms a metal titanate substrate.